The Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA) is an area of the rat hypothalamus which is sexually different in terms of volume and neuronal content. Moreover, the volume of the SDN-POA is influenced by the perinatal steroidal environment. The neuroblasts which eventually give rise to the neurons of the SDN-POA will be labelled specifically with tritiated thymidine and neuronal migration and survival evaluated over time through the technique of autoradiography. In addition, the influence of the direct injection of gonadal steroids (testosterone, estradiol, or combinations thereof) to the rat fetus or newborn pup on the development of the SDN-POA will be ascertained. The SDN-POA of newborn male rats will be grafted into the hypothalamus of the newborn female and possible alterations in neuroendocrine function evaluated in the adult recipient. Finally possible efferent and afferent connections of the neurons of the SDN-POA will be studied. As a distinct morphological signature of the action of steroid hormones on the developing brain, the SDN-POA can serve as a model system for the elucidation of the process of the functional sexual differentiation of the brain.